In recent years, development of a technique of using a storage cell as a power source for electric vehicles, hybrid vehicles, or the like is carried out. Further, there is also carried out development of a technique of using such a storage cell as a distributed power source for power adjustment, as exemplified by a solar battery, wind power generation, or the like, in which the storage cell is incorporated in a power system.
Regarding a power source for power adjustment, durability of ten years or longer is required, and reduction of the production cost and the maintenance cost is required. It is important to extend the product lifetime in order to meet these requirements.
A best method for extending the product lifetime is optimization of a material or a structure of a battery cell constituting a storage cell. However, development of this technique requires a large amount of time and money. In view of the above, it is important to carry out development of a technique of suppressing lifetime degradation of a storage cell concurrently or prior to development of a technique of extending the product lifetime.
Note that lifetime degradation is associated with degradation of performance of supplying the amount of power storable by a storage cell or stored power. In view of the above, in the following description, degradation of lifetime of the product is described as capacity degradation as necessary.
Capacity degradation of a storage cell greatly differs depending on an operation method or the like. In view of the above, for instance, PTL 1 or PTL 2 proposes a technique of suppressing capacity degradation by improving a charge/discharge method with respect to a lithium ion secondary battery.
Specifically, in PTL 1, charge/discharge of a lithium ion secondary battery is controlled in such a manner that the amount of lithium ions migrating between a cathode active material and an anode active material is equal to or less than 95% of the amount of reversibly movable lithium ions.
Further, in PTL 2, charge/discharge of a lithium ion secondary battery is controlled in such a manner that the end-of-discharge voltage during discharge is set to 3.2 to 3.1 V, and the upper limit voltage of a cell during charge is set to 4.0 to 4.5 V.